The feat seems to be custom tailored for characters like the Hexblade, and would probably be part of my hexblade's feat collection. Just seems logical.

Or a Paladin or Ranger -- melee types who are already provides with spell-slots they may have no use for in a given adventure. A ranger with several Entangle spells who suddenlyfinds himself in a vegetation-less environment will greatly appreciate the ability to turn each of those Entangle spells into +1 to hit and +1d4 damage for a Full Attack action each.

Um. Does the feat say it has to be used with a WEAPON, or can you use it with a touch spell ... ? If touch spells are okay, picture a sorceror burning an extra 2d or 3d level slot to boost their chill touch full attack routine for one round. ^_^

Or a Paladin or Ranger -- melee types who are already provides with spell-slots they may have no use for in a given adventure. A ranger with several Entangle spells who suddenlyfinds himself in a vegetation-less environment will greatly appreciate the ability to turn each of those Entangle spells into +1 to hit and +1d4 damage for a Full Attack action each.

Just remember that the feat only affects one weapon. A dragon could boost one claw but not two (at least not without dropping multiple spell slots).

So he may not want to use it during a full attack, but in situations where a single attack only is available it is better. Of course, he may wish to use it in a full attack situation ...

Compare the following:

The expected normal attack damage of an ancient red wyrm using a full attack action and optimal power attack versus an AC 40 target vs the expected normal attack damage damage for an ancient red wyrm using a single attack versus that same foe, arcane strike (9th level) and optimal power attack ... especially when it has great cleave. That dragon might be far better off doing an insane amount of damage in one attack and relying upon cleaving to take out all of his foes in one round. A single attack that does lots of damage can be far better than a collection of attacks that each do less damage.

Also keep in mind, nothing stops the dragon from forgoing it's normally-advantageous NaturalAttack Progression in favor of an Iterative progression with a single natural weapon. Four bites (five with Haste going), in one round, all of them at +9 to hit and +9d4 damage 'cause the wyrm burns a 9th level spell through the Arcane Strike feat ... ?

That puts you on a trip straight to lovely downtown OUCH-city, my friends. On the Express bus!